1. Field of the Invention
This invention relates to an electromagnetic device for use in, for example, the diaphragm control of a camera.
2. Description of the Prior Art
An example of a conventional device of this kind is shown in FIGS. 1 to 5, including an armature 1, a yoke 2, a pair of coils 3, and an armature shaft 4 with a flange 5 mounted on an armature lever 7. The shaft 4 has a pair of short diameter portions 4c on either side of a large diameter portion 4b about which the armature 1 is pivotable in the directions of arrows D (see FIG. 2) to allow intimate contact of a reactive surface 1a of the armature 1 with attraction surfaces 2a of the yoke 2 over the entire area thereof. An E type retainer ring 6 is pressed into an annular groove 4a of the shaft 4 after the armature 1 has been installed on the shaft 4. The armature lever 7 is pivotally mounted about a pin (not shown) in a fitted hole 7a and is urged by a spring 8, in most cases, in a counterclockwise direction. Reference numeral 9 identifies a charge member, and 10 identifies a member to be driven, for example, identifies diaphragm stop member in the shutter priority camera.
As the spring 8 usually sets the armature lever 7 in the initial or most counterclockwise position where the armature 1 is separated from the yoke 2, after the charge member 9 moves the armature lever 7 in the clockwise direction of arrow A (see FIG. 1) until the armature 1 contacts the yoke 2, when the coils 3 are supplied with current, the electromagnetic force exerted in the yoke 2, despite the return of the charge member 9 to the initial position of FIG. 1, must hold the armature 1 in contact with the yoke 2. In desired timing, the current supplied to the coils 3 is then cut off, permitting the armature lever 7 to turn in a counterclockwise direction by the bias force of the spring 8, whereby the member 10 moves. By the way, in FIG. 2, the reactive surface 1a of the armature 1 and the attraction face 2a of the yoke 2 show a parallel or ideal relationship. With the electromagnetic device installed in such a relationship, when the armature 1 is brought into mechanical contact with the yoke 2, the reactive surface 1a of the armature 1 arrives at the upper and lower edges 2b and 2c of the yoke 2 simultaneously. Therefore, a stable attraction can be established when the coils 3 are energized.
Due to the tolerances of the parts, however, it is very rare that the spatial relationship of FIG. 2 is set up. In most cases, the plane of the armature lever 7 tilts either clockwise or counterclockwise so that the reactive surface of the armature 1 and the attraction surface 2a of the yoke 2 are not parallel with each other as shown in FIGS. 3A and 4A. In FIG. 3A where the axis of the shaft 4 is inclined clockwise from the parallel position with the attraction surface 2a, or the distance from the reactive surface 1a to the upper edge 2b is longer than that to the lower edge 2c, when the armature lever 7 is turned clockwise in FIG. 1 (to the left in FIG. 3A), the reactive surface 1a first contacts with the lower edge 2c, and, as the lever 7 is pushed further, the armature 1 is then pivoted about the edge 2c in the direction of arrow B until the reactive surface 1a rests on the entire area of the attraction surface 2a. Then, when the coils 3 are energized, the armature 1 can be reliably held in the position of FIG. 3B by attraction. It should be noted here that a point G on the axis of the shaft 4 does not shift vertically during the time between before and after the attraction or the positions of FIGS. 3A and 3B. This implies that the armature lever 7 does not distort, because the pivotal arrangement of the armature 1 about the large diameter portion 4b of the shaft 4 absorbs the discrepancy of the actual angular position of the lever 7 from the ideal position of FIG. 3A.
In FIG. 4A, on the other hand, the reactive surface 1a first contacts with the upper edge 2b of the yoke 2. For perfect contact to be established, the armature 1 must then turn about a line 0 perpendicular to the paper (in contact with the upper edge 2b) in a clockwise direction of arrow C (see FIG. 4A). But this is not permitted, because the bottom surface 1b of the armature 1 rests on the flange 5. As the lever 7 is pushed further, therefore, twisting of the lever 7 takes place, as shown by the difference in the height h of a point G before and after perfect contact is reached or the positions of FIGS. 4A and 4B respectively. The electromagnetic force is generally not strong enough to overcome the recovering force of the twisted lever 7. Therefore, soon after the charge lever 9 has moved away from the lever 7, the armature 1 turns backward about the line 0, assuming the attitude of FIG. 4A with its upper end only in contact with the yoke 2 at the upper edge 2b. The resultant attracting state is very unstable so that when any shock takes place, such an imperfect contact will be broken easily.
From the above it will appear that the abovedescribed problem can be solved if all the parts are assembled to one-sided spatial relationships, as typically represented by the position of FIG. 3A by taking into account the tolerances of the parts. Even when the electromagnetic device has such a characteristic, however, when turned upside down (for example, the camera employing it is held with its bottom pointing upward), the position of FIG. 3A turns to a position of FIG. 5, where similar to the position of FIG. 4A, the reactive surface 1a contacts only the upper edge of the yoke 2 (or the lower edge 2c as viewed in FIG. 3A), and the clockwise movement of the armature 1 about the edge 2c is barred by the retainer ring 6.
It should be recognized that if only setting up the armature lever 7 to the clockwise orientation is relied upon, it is impossible to assure proper holding of the armature 1 by the yoke 2 in all angular positions of the electromagnetic device.
An object of the present invention is to provide an electromagnetic device capable of maintaining the attraction in a state stable condition in all angular positions.